In the drilling of oil and gas wells by the rotary method, drilling fluid, commonly called "mud", is used to remove drill cuttings from the well. The mud circulates down through a drill string and out a drill bit at the lower end of the drill string and then circulates up through the wellbore to the earth's surface. Drill cuttings are removed from the mud by solids control equipment such as shale shakers and hydrocyclones, and the mud is recirculated back into the wellbore.
As the well depth increases, so does the earth's pressure. For effective well control in deep wells, the mud must be weighted with materials having a high specific gravity to prevent unwanted entry of formation fluids into the wellbore. Known weighting materials include barite, galena, lead oxide, barium carbonate, and iron oxide. Naturally occurring iron oxide, principally hematite (Fe.sub.2 O.sub.3), was among the first materials used in the oil and gas industry to increase the mud density. However, use of iron oxides was largely discontinued in the early 1940s when ample supplies of barite became available. While barite continues to be the most common weighting material for drilling fluids, as the world's supply of barite dwindles, the use of iron oxide is increasing.
Since large quantities of weighting material are needed in drilling an oil well, it is desirable to recover the material and recycle it. Various solids control systems are in use today for separating the drill cuttings from the mud so that the mud's liquid component and the mud's weighting material can be recycled, leaving only drill cuttings for disposal. Most systems use a combination of one or more screens or sieves in a series relationship, with a final separating step using a mud cleaner or one or more centrifuges.
When hematite is used as the weighting material, magnetic separators have been proposed for separating the paramagnetic hematite from the mud. For example, SPE Paper Number 4517, entitled "A New Weighting Material for Drilling Fluids Based on Synthetic Iron Oxide" (presented at the 48th Annual Fall Meeting of the Society of Petroleum Engineers in 1974) proposed using a wet magnetic separator for removing a hematite-based weighting material. The SPE paper further suggests that in place of a magnetic separator a centrifuge separator can be used to remove the weighting material. There is no suggestion in the SPE paper of using a magnetic separator and centrifuge separator in combination.
As the cost of waste disposal increases, there is a growing incentive to reduce the volume of drilling waste. One way of reducing this volume is to reduce the amount of liquid discharged with the drill cuttings. A mud treating system's effectiveness is measured by the amount of liquid recovered for reuse in the drilling operation. Ideally, a solids control system discharges the drill cutting in a dry state, but most systems in use today discharge substantial volumes of liquid with the drill cuttings.
While many solids control systems have been used in the oil and gas industry, there continues to be a need for a more efficient system for recovering weighting materials and at the same time removing drill cuttings with a minimum amount of liquid.